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**DGXSER** · 04-19-2013, 08:57 AM

Hey George,

The theory behind the holes is exactly as you have it outlined. The pilot air jet in the inlet bell determines how much air is brought to the pilot jet. The air holes, the ones in the sides, of the pilot jet determine how much is mixed with the fuel being metered through the pilot jet opening to the float bowl to form the air fuel mixture that leaves the jet and gets fed into the air stream in the main path of the carb. There are at least three styles that I know of, the 6 hole BS30/96 style, the eight hole VM style, and the zero hole N151 style.

From what I have seen, and experienced working on a couple sets of carbs where folks have had the wrong jets, no amount of playing with the mixture screws will adjust it to run correctly if at all if ti has the wrong style of jets in it.

Now, the good folks at Mikuni know a HElluva lot more than I do about carb design. So, while you may be able to change out the air jet to lean things out enough to match the pilot fuel jet with to many holes in it to get the right ratio of fuel to air, have you actually achieved the right amount of fuel air mixture for the rest fo the carb components? I have my doubts you will actually get a good mixture to help your bike run at its optimum performance level. Besides, once you are down to changing jets, why not change the pilot fuel jet to the correct one for that carb to work properly on the XS11 engine.

Now for the 78-79 model carbs, I have seen first hand the number of holes be just as detrimental to their performance, just as I have first hand knowledge on the new style carbs with the wrong pilot jets. If your referring to the early 80 model carbs that had the new body style but still the crossover, I can not speak from experience on those, as I only saw and worked on one set. And that was on a bike with alot more issues, so we never really got it running right before it got burned up.

Hope that gives you a little more thoughts on it.

**GLoweVA** · 04-19-2013, 09:29 AM

Thanks, that's exactly the insight I was looking for.

I thought the difference between the BS and VM style was where the metering orfice was located.

But if you look at the 8 holed jets, the metering hole is near the tip, not the threads like the VM style jet.

Sorry I don't have any pics showing the metering end, but to show all of you the 3 different kinds,

Here's the no holed one:

And a pic of both the 6 and 8 holers:

**GLoweVA** · 04-19-2013, 09:41 AM

Ok, so here's a quote from a member back in 2005:

Originally posted by Dan Hodges View Post

This may or may not help,but I bought my 81 SH new and only I have touched it and mine had/has #42.5 pilots with no side holes and no plugs on top.I have one set of new pilots that I've had for about 5 years and the bag is numbered 4GO-14142-42-AO.I also have a set of #45 & 47.5 pilots from Yamaha that are of same configeration ,i.e. no side holes.I have a new rack of carbs for an 80 standard and they have #42.5 pilots that are capped with rubber plugs and the pilots have side holes,but I do not recall how many holes.It is my understanding that carbs that were equipped with capped pilots that feed from the mains had pilots with side holes and those that were not capped (81 SH),did not have the pilots with side holes.

Here's that part number: (with no holes on the side)

So, now what?....

**natemoen** · 04-19-2013, 10:18 AM

Because it wasn't designed to use those jets. Mikuno pit the 6 hole in these carbs because that is what there overly smart engineers designed these carbs to use. Plenty here have fought for years trying to make their bike work right all to eventually find out they had the wrong jet.

This horse is dead!

**GLoweVA** · 04-19-2013, 10:23 AM

Sorry to beat the dead horse Nate!

I'll let it go for now and let my mind mull it over some more as well as look deeper at my carb rack and see if I can enlighten myself.

**natemoen** · 04-19-2013, 10:44 AM

It's not like the 6 hole are nla, so why bother. Get the right ones and be done.

Not trying to be a dick...

**DGXSER** · 04-19-2013, 11:57 AM

George, as an engineer it is engrained in me to know why something is the answer and not just what the answer is, so I feel your angst over the confusing information. I can tell you that Dan Hodges is a very smart and knowledgeable guy. Have not seen him around for years, but he was active when I first came to the site. Seems like he has done and tried everything known to man or beast to get 100 RWHP without going to a turbo. If he syas his carbs came form the factory with the N151 jets, well, consider it the same as Moses carrying it down from the mount.

I do find it interesting that he states his factory carbs had the N151 style pilots in them. I know this does not answer your question, but I have worked on a set of carbs that a member had those N151 style jets in, and it ran like absolute crapola till we swapped them out for the BS30/96 style. I have a couple sets of 81 carbs with the BS30/96 jest in them that run very well.

I had not recalled the change in the metering port as shown in your cutaway. That does bring up an interesting point. As with the meteirng point for fuel prior to the air coming in, with the eight holes, you are metering raw fuel that then gets air added in with the only thing controlling the volume of mixture to the carb being the number of holes. (More air, more volume in the same amount of raw fuel.) Move the metering after the air holes and now your metering the mixture. Since we know the 42.5 is based on the BS30/96 style jets, you would now need to determine the correct size metering hole to get the same mixture out of the VM style jets. which could easily be a different size.

It appears I have no answers, only questions. And the only real answer I can give you is to keep cutting the ends off the meatloaf.

**GLoweVA** · 04-19-2013, 01:57 PM

I like the ends of the meatloaf!

Seeing as I have all 3 jets (either in my hand or in my bike, but not enough to swap all 4

) I'm going to try some 'theory to practice' as it's called and see if I can sort of be a little scientific about this.
Plus I also have both styles of the later carbs. (one with the crossover and one with out)
So, with these variables I should be able to figure this out as to why mother Yamie lists 4GO-14142-42-AO for those non holed pilots for the SG.

Maybe it was a emissions thing and they came with the 6 holes, but then later swapped to the non holed ones.
I'm pretty sure all 3 of these different types that I got are BS types with the metering hole near the tip, but I'll double check this weekend for sure and post an update with more info.

**GLoweVA** · 04-19-2013, 05:40 PM

pics

Okay, here are all three:

You can see all three have the metering holes near the top.

Then I decided to see how 'deep' the inlet hole went to see if the others' metering orfice was closer to the inlet.
I used a small allen the just fit inside the inlet hole and all 3 went the same depth.

Now, I did notice that my 8 holed one has a little bit bigger orfice. It is listed as a 42 on the side whereas the 6 holed one is a 42.5 (as we all know) and the non-holed one I ordered with the yammie part number (ordered a 42.5) both have the same diameter/sized orfice.

So, if the 8 holed ones do have a biger hole then yes, they would mess with the mix and make the bike more richer and it would run crappier.

My next step is to pull my SG carbs off and really inspect the pilot tower to see where the air inlet is and maybe pull the carbs off the G and see if the crossover makes a difference.

Stay tuned, more pics to come.

**Rasputin** · 04-19-2013, 07:55 PM

Originally posted by GLoweVA View Post

Okay, here are all three:

You can see all three have the metering holes near the top.

Then I decided to see how 'deep' the inlet hole went to see if the others' metering orfice was closer to the inlet.
I used a small allen the just fit inside the inlet hole and all 3 went the same depth.

Now, I did notice that my 8 holed one has a little bit bigger orfice. It is listed as a 42 on the side whereas the 6 holed one is a 42.5 (as we all know) and the non-holed one I ordered with the yammie part number (ordered a 42.5) both have the same diameter/sized orfice.

So, if the 8 holed ones do have a biger hole then yes, they would mess with the mix and make the bike more richer and it would run crappier.

My next step is to pull my SG carbs off and really inspect the pilot tower to see where the air inlet is and maybe pull the carbs off the G and see if the crossover makes a difference.

Stay tuned, more pics to come.

Not seeing any pic's myself but maybe my connection is bad.

**petejw** · 04-20-2013, 01:52 AM

hi George,
ive had those jets before and as u mentioned
the size of the orifice is larger I measured it
at .50 much using jet bits,
bigger than 42.5 which it should be.

**GLoweVA** · 04-21-2013, 10:47 AM

More Pics

Okay, so the 8 holed ones have too large of a metering orfice and those won't work without some other tuning.
But what I did find out is the non holed ones that Yammie sent me will work.

Get ready to change the way you think about the pilot circuit.

So, I put a regular 42.5 6 holer in the pilot tower and spray carb cleaner into the pilot air jet and this is what I get:

I put a non holer 42.5 in the other tower and try the same test and ....

SAME RESULTS!
So the air mixes after the metering orfice!

I put the 2 non holed jet that I do have into #1 and 4 of my SG and she fired right up and tuned just fine.

In conclusion: the 8 holed ones (that have a bigger orfice) WON'T work, but the non holed ones will.

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